# LORAN C

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```					                        LORAN C

Became feasible for aircraft navigation with the introduction
of microprocessors

Frequency of Operation: 100kHz (all stations)
LORAN C
• A HYPERBOLIC SYSTEM

i.e. lines of position are hyperbolas

• This results from the fact that the lines of position are
determined by measuring the DIFFERENCE in
distance from two points.
LORAN C

One station is referred to as the Master and the others as Slaves
LORAN C

• At least two lines of position are required
for a position fix thus more than one slave is
required
LORAN C

• A useful property of the hyperbola is that its
tangent at any point bisects the angle subtended by
the line joining the two foci
• Exercise: Use this property to determine where the
best geometry occurs (LOP at 90º)
LORAN C
• How do we determine the time difference?
• Each station, starting with the Master,
transmits a series of pulses with the
following shape:

This pulse has a
20kHz
LORAN C

• Each station transmits a series of eight of these
pulses
• Pulse separation is 1000μs (1ms)

Note: In most chains the master transmits a ninth
pulse after 2000μs. This can be used to indicate
the status or integrity of the chain’s signals
LORAN C
How do we identify the pulses from each
station?
• The stations transmit their signals in
sequence. The delay between signals from
each station is such that the signal from the
previous transmission is out of the coverage
area before the next is sent.
• Thus they always appear in the same order
LORAN C Chains

• A group consisting of a Master and up to
four slaves is called a chain
• Each chain is identified by a Group
Repetition Rate (GRI) which is the time
between transmissions from the master.
LORAN C Chains

• Each slave transmits its pulse train at a specified
interval after the master has transmitted.
• This is called the emission delay (ED) and is made
up of the master-slave time (MS) and a coding
delay (CD)
LORAN C Transmitters
• Due to the long distances covered by each
LORAN C chain, the power transmitted must be
high (0.5 to 4 MW)
• Propagation is by ground wave and thus has to be
vertically polarized
• Antenna therefore is a vertical mast (ideally a
quarter wavelength long (3km) (10,000 ft.)
• Not very practical!!
LORAN C Antennas
• Antennas are typically about 400m high
• To improve the current flow, many are “top
• They are still not very efficient (~10%)
LORAN C Antennas

“Top loaded” antenna with ground plane
•Receivers require a data base which provides
• the location (Lat/Lon) of the Master and Slave stations
• the GRI of the chains to be used
• the Time Delays for the individual stations
•The LORAN C signal travels both by ground wave and sky wave
• ground wave gives stable, reliable timing
• sky wave does not due to the variable nature of the
ionosphere
• ground wave is attenuated more and hence is weaker and can
be contaminated by the sky wave
• Since sky wave is always delayed by a minimum of 30μs, the
positive-going zero crossover of the third cycle of the ground
wave is used for timing
•Problems to be solved by receiver
•Signals strength may vary by 120dB
•Large dynamic range required
•Noise at LF can be very high due to long range
propagation of interference (e.g. lightning in tropics)
•Signal to noise ratio can be – 20 dB
•Searches for Master pulses using known GRI
•PLL locks on to carrier to generate master clock
•Locks on to slave pulses
•Measures Master/slave time interval and subtracts the
Emission Delay (ED)
•Calculates the distances and position
Phase Locked Loops (PLLs)
LORAN C Accuracy
Error Sources
Variation in propagation speed (land vs water, type of terrain)
Changes in signal strength

• Absolute Accuracy depends on geometry
•0.1 to 0.25NM
• Repeatability
•20 to 100m
LORAN C
• Integrity
•Monitors are installed throughout the LORAN C
coverage area
•These monitors adjust the transmitter timing to
compensate for changing propagation conditions
•If excessive errors are detected, the master
transmitter is commanded to “blink” the ninth pulse
off and on to indicate which station is unreliable
•For airborne use, this can be done within 10 seconds
of detection
•
LORAN C Coverage

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